The invention concerns a method for contact-free drying of a paper or board web, or of any other corresponding continuous web, in which method both infrared radiation and drying air jets are used for drying, the air jets also supporting the web as it runs through the dryer, so that the web is carried free of contact, preferably from two sides, and in which method, after the infrared drying gap, the web is substantially immediately passed into an airborne web-drying gap, wherein the web is supported and dried by means of air jets.
Also included herein is a description of a device intended for carrying out the method of the invention, which device comprises an infrared drying unit and an airborne web-drying unit or airborne web-drying units, which infrared drying unit comprises a series of infrared radiators and an infrared treatment gap fitted in its connection, through which gap the web to be dried can be passed. The airborne web-drying unit or units comprise a box portion, inside of which a nozzle box or boxes are fitted, in connection with which there are nozzle parts, through which drying and supporting air jets are applied to the web to be dried. The infrared drying unit and airborne web-drying unit are integrated with each other both structurally and functionally, and the infrared unit is placed, in the direction of running of the web to be dried, immediately before the airborne web-drying unit.
The present invention relates to the drying of a paper web, board web, or of any other corresponding moving web. A typical object of the invention is the drying of a paper web in connection with its coating or surface-sizing.
As is known in the prior art, paper webs are coated either by means of separate coating devices or by means of on-machine devices or surface-sizing devices integrated in paper machines and operating in the drying section of a paper machine. At the final end of a multi-cylinder dryer, the web to be coated is passed to a coating device, which is followed by an intermediate dryer and finally, e.g., by one group of drying cylinders as an after-dryer. A typical application of the present invention is the intermediate dryer after the coating device, the invention being, however, not confined to the intermediate dryer alone.
In the prior art, so-called airborne web dryers are known, wherein a paper web, board web, or equivalent is dried free of contact. Airborne web dryers are used, e.g., in paper coating devices after a roll coater or a spread coater to support and to dry the web, which is wet with the coating agent, free of contact. In airborne web dryers various blow nozzles and nozzle settings for drying and supporting air are applied. The blow nozzles can be divided into two groups, i.e. pressure or float nozzles, and negative-pressure or foil nozzles, both of which can be applied in the dryer and the method in accordance with the invention.
The prior art airborne web dryers that are used most commonly are based exclusively on air flows. It is partly for this reason that the airborne web dryer becomes quite spacious, since the distance of effect of the airborne web dryer must be relatively long in order that a sufficient high drying capacity could be obtained. Another reason for these drawbacks is that in air drying the depth of penetration of the drying remains relatively low.
In the prior art, different dryers are known which are based on the effect of radiation, in particular of infrared radiation. The use of infrared radiation provides the advantage that the radiation has a relatively high depth of penetration, which depth of penetration is increased when the wavelength becomes shorter. The use of infrared dryers in the drying of paper webs has been hampered, e.g., by the risk of fire, because the temperatures in infrared radiators become quite high, e.g. 2000.degree. C., in order that a drying radiation with a sufficiently short wavelength could be achieved.
With respect to the prior art, reference is made to the German published Patent Application (DE OS) No. 2,351,280, which describes a sort of a combination of an airborne web dryer and an infrared dryer operating by means of pressure nozzles. In the patent application mentioned above, a one-sided airborne web dryer is described, which comprises nozzle boxes placed one after the other at distances from each other. The edge portions of these boxes are provided with nozzle slots, through which air jets are directed at the web placed above expressly perpendicularly. The air jets are deflected outward from the nozzle box when they meet the web. Between the nozzles, infrared radiators are fitted, which fill the gap between the nozzles. This type of dryer has not become widely used, probably due to the fact that the nozzle construction has not been successful in providing a constructionally or energy-economically favorable combination of air drying and radiation drying. Moreover, the construction is one-sided, and it requires a relatively abundant space in the direction of running of the web if sufficiently high drying capacities are to be reached, e.g., in paper finishing plants.
Particular problems in infrared drying have been the strong formation of dust and high humidity of air.
Electric infrared dryers, used separately or exclusively, are also energy-economically unfavorable owing to the relatively high cost of electric energy, as compared, e.g., with natural gas.
In paper coating stations, including on-machine coating stations, separate infrared dryers have been used whose drying is based exclusively on the radiation effect. However, use of these infrared dryers has not yielded a sufficiently good adjustability of paper quality and evaportion. Moreover, the drying process becomes highly dependent on the operating quality of the infrared dryer.
It is an object of the present invention to solve the problem described above.
It is a particular object of the present invention to develop a novel application of an infrared dryer, in which the air technique particularly has been solved in a better way than in the prior art.
A further object of the invention is to provide a method and to describe a device by means of which the overall control of the coating-drying of a paper web can be improved.
Another objective of the invention is to provide a novel application of an infrared dryer so that it is possible to attain a dryer with more favorable investment costs and operating costs, as compared with the prior art. In view of achieving this objective by means of the invention, attempts are made to obtain a higher drying capacity, a lower size of equipment, and a lower heat and humidity load in the machine hall.
It is a particular object of the invention to provide an infrared dryer that can be used for adjusting the ultimate moisture profile of the web produced by the paper machine.
In view of achieving the objectives given above, as well as others, the method of the invention is mainly characterized as follows:
the moving web is first passed into an infrared drying gap in which a drying energy pulse of relatively short duration is directed at the web, the power of the energy pulse being substantially higher than the average drying power of the dryer per unit of area, and PA1 air is brought into the infrared unit, which air, having been heated in the infrared unit, is passed as replacement air and/or drying air for the airborne web-drying unit or units placed after the infrared unit.
On the other hand, the drying device in accordance with the invention is mainly characterized in that the infrared unit comprises air and nozzle devices, through which air flows can be passed into the treatment gap of the infrared unit and/or in connection with the heated parts of the infrared unit, which air flows are passed for replacement and/or drying air for the subsequent airborne web-drying unit or units.
By means of the invention, it is possible to accomplish drying with improved overall profitability, wherein both the investment costs and the operating costs are taken into account.
Owing to the invention, an increased evaporation capacity, a reduced heat and humidity load in the machine hall, as well as economies in the lifting and auxiliary equipment for the infrared dryer are obtained. On the basis of measurements, drying test runs, and theoretical examinations carried out by the inventor, it has been ascertained that the solution of the invention, from an evaporation standpoint, and also in view of the quality of the paper web, results in a considerable improvement over the prior art dryer arrangements in which the infrared dryer and the airborne web dryer are provided as separate, independently operating units.
The method and device in accordance with the invention are particularly well suited for an on-machine dryer after a coating or surface-sizing apparatus and moreover, if necessary, also for adjustment of the ultimate moisture profile of the paper web.
In the present invention, an open hood does not have to be constructed above the dryer, which is the case in the prior art devices, for in the infra-airborne combination of the invention mere spot exhaustion is enough, because the system of exhaust ducts in the airborne web dryer provides adequate ventilation.
When natural gas or a corresponding fuel is used for the heating of the drying air for the airborne web dryer unit or the heating of the drying air for the airborne web dryer unit or units, the operating cost of the method and the device making use of the invention per unit of quantity of evaporated water becomes considerably more favorable as compared with a dryer in which electric infrared drying along would be used. This advantage is due to the fact that in the invention the energy transferred into the paper web in the electric infrared unit is utilized efficiently in the airborne web drying unit or units following after the infrared unit.